The present invention provides certain improvements in planarizing workpieces. The invention has particular utility in connection with conditioning CMP pads, though it may also be used in other applications, such as in planarizing semiconductor wafers or other microelectronic workpieces.
Mechanical and chemical-mechanical planarizing processes (collectively “CMP processes”) remove material from the surfaces of semiconductor wafers, field emission displays, or other microelectronic/workpieces in the production of microelectronic components and other products. FIG. 1 schematically illustrates a planarizing machine 10 with a circular table or platen 20, a first carrier assembly 30, a planarizing pad 40 having a planarizing surface 42, and a planarizing fluid 44 on the planarizing surface 42. The planarizing machine 10 may also have an under-pad 25 attached to an upper surface 22 of the plate 20 for supporting the planarizing pad 40. A drive assembly 26 rotates the platen 20 (indicated by arrow A) and/or reciprocates the platen 20 back and forth (indicated by arrow B). Since the planarizing pad 40 is attached to the under-pad 25, the planarizing pad 40 moves with the platen 20 during planarization.
The first carrier assembly 30 has a carrier head or substrate holder 32 with a pad 34 that holds the workpiece 12 to the carrier head 32. An actuator assembly 36 may be coupled to the carrier head 32 to impart axial and/or rotational motion to the carrier head 32 (indicated by arrows C and D, respectively). The carrier head 32, however, may be a weighted, free-floating disk (not shown) that slides over the polishing pad 40. The carrier head 32 may be coupled to a sweep actuator 33 by an arm 31. The sweep actuator 33 may rotate the arm 31 (indicated by arrow E) to reciprocate the carrier head 32 along an arcuate path across the planarizing surface 42.
The planarizing pad 40 and the planarizing solution 44 collectively define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the workpiece 12. The planarizing machine 10 can use a fixed-abrasive planarizing pad 40 having abrasive particles fixedly bonded to a suspension material. The planarizing solutions 44 used with fixed-abrasive pads are generally “clean solutions” without abrasive particles. In other applications, the planarizing pad 40 may be a nonabrasive pad composed of a polymeric material (e.g., polyurethane), a resin, felt, or other suitable material without abrasive particles. The planarizing solutions 44 used with nonabrasive polishing pads are typically abrasive slurries that contain abrasive particles suspended in a liquid.
If chemical-mechanical planarization (as opposed to plain mechanical planarization) is employed, the planarizing solution 44 will typically chemically interact with the surface of the workpiece 12 to speed up or otherwise optimize the removal of material from the surface of the workpiece. Increasingly, microelectronic device circuitry (i.e., trenches, vias, and the like) is being formed from copper. When planarizing a copper layer using a CMP process, the planarizing solution 44 is typically neutral to acidic and includes an oxidizer (e.g., hydrogen peroxide) to oxidize the copper and increase the copper removal rate. One particular slurry useful for polishing a copper layer is disclosed in International Publication Number WO 02/18099, the entirety of which is incorporated herein by reference.
To planarize the workpiece 12 with the CMP machine 10, the carrier assembly 30 presses the workpiece 12 face-downward against the polishing medium. More specifically, the carrier assembly 30 generally presses the workpiece 12 against the planarizing solution 44 on a planarizing surface 42 of the planarizing pad 40, and the platen 20 and/or the carrier assembly 30 move to rub the workpiece 12 against the planarizing surface 42. As the workpiece 12 rubs against the planarizing surface 42, material is removed from the face of the workpiece 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate assembly 12 to enable precise fabrication of circuits and photo-patterns. For example, during the fabrication of transistors, contacts, interconnects and other components, many substrate assemblies develop large “step heights” that create a highly topographic surface across the substrate assembly 12. To enable the fabrication of integrated circuits with high densities of components, it is necessary to produce a highly planar surface at several stages of processing the substrate assembly 12 because non-planar surfaces significantly increase the difficulty of forming submicron features. For example, it is difficult to accurately focus photo-patterns to within tolerances of 0.1 micron on nonplanar surfaces because submicron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes often transform a topographical surface into a highly uniform, planar surface.
In the competitive semiconductor industry, it is also desirable to have a high yield of operable devices after CMP processing, yet maximize throughput by producing a planar surface on a workpiece 12 as quickly as possible. CMP processes should thus quickly remove material from the substrate assembly 12 to form a uniformly planar surface at a desired endpoint. For example, when a conductive layer on the substrate assembly 12 is under-planarized in the formation of contacts or interconnects, many of these components may not be electrically isolated from one another because undesirable portions of the conductive layer may remain on the substrate assembly 12. Additionally, when a substrate assembly 12 is over-planarized, components below the desired endpoint may be damaged or completely destroyed. Accurately stopping CMP processing at a desired endpoint helps maintain high yield, high throughput operation because the workpiece may need to be re-polished if it is “under-planarized,” or components on the workpiece may be destroyed if the workpiece is “over-polished.”
In one conventional method for determining the endpoint of CMP processing, the planarizing period of a particular substrate is fixed using an estimated polishing rate based upon the polishing rate of identical substrates that were planarized under the same conditions. The estimated planarizing period for a particular substrate, however, may not be accurate because the polishing rate or other variables may change from one substrate to another, from one lot of consumables to another, or even from one day to another. Thus, this method may not produce accurate results.
One variable affecting the polishing rate and uniformity of microelectronic workpieces is the condition of the planarizing pad 40. Hence, one aspect of CMP processing is establishing and maintaining the condition (both uniformity and roughness) of the planarizing surface 42 on the planarizing pad 40. Most planarizing pads 40 are initially received from the manufacturer with a hydrophobic, non-planar surface. Before the planarizing pad 40 is used to planarize a microelectronic workpiece 12, the pad 40 is initially conditioned or “broken in.” The parameters of the break-in process are typically derived from extensive trial and error. Any changes in these empirically-derived parameters from one pad to the next can adversely impact subsequent planarization processes.
The condition of the planarizing surface 42 also changes over time because residual matter collects on the planarizing surface 42 of the planarizing pad 40. The residual matter, for example, can be from the workpiece 12, the planarizing solution 44 and/or the planarizing pad 40. In certain applications, residual matter from the workpiece 12 can even glaze over sections of the planarizing surface 42 (e.g., planarizing doped silicon dioxide layers). The workpieces 12 can also wear depressions into the planarizing surface 42 that create a non-planar planarizing surface. In many CMP applications, therefore, planarizing pads 40 are accordingly “conditioned” periodically to bring the planarizing surface 42 into a desired condition for planarizing the workpieces 12.
Planarizing pads 40 may be conditioned using a “conditioning stone” or “conditioning pad.” In some operations, the planarizing pad 40 is removed from the platen 20 and placed on a separate conditioning machine (not shown). The planarizing machine 10 of FIG. 1, however, includes a conditioning system 50 that rubs an abrasive conditioning stone 60 against the planarizing surface 42 of the planarizing pad 40 between planarizing cycles. The conditioning stone 60 typically includes a second carrier head 62, a bonding layer 64 of nickel or the like covering the bottom surface of the second carrier head 62, and a plurality of diamond particles embedded in a conditioning surface 66 of the bonding layer 64.
The second carrier head 62 is part of a second carrier assembly 70 that sweeps the conditioning stone 60 over the planarizing pad 40 and presses the conditioning surface 66 against the planarizing surface 42. The second carrier assembly 70 of FIG. 1 includes an actuator assembly 74 coupled to the carrier head 62 and to an arm 72. The actuator assembly 74 can rotate the carrier head 62 (indicated by arrow G) and/or move the carrier head 62 axially (indicated by arrow F) to selectively engage the conditioning surface 66 with the planarizing surface 42 and control the force with which the conditioning surface 66 acts against the planarizing surface 42. The second carrier assembly 70 may also include a sweep actuator 76 which rotates the arm 72 (indicated by arrow H) to reciprocate the second carrier head 62 along an arcuate path across the planarizing surface 42.
One problem with conventional conditioning stones 60 is that they wear out over time. Most conventional conditioning systems 50 rub the conditioning stone 60 against the planarizing pad 40 for a fixed period of time. As the conditioning stone 60 degrades, it will remove less of the planarizing pad 40. This leads to variations in the condition of the planarizing pad 40, which can adversely impact quality control of workpieces 12 planarized with the polishing pad 40. At some point, the conditioning stone will no longer remove enough of the planarizing pad 40 in the fixed period of time to appropriately recondition the planarizing surface 42 to the desired uniformity and roughness. Such a conditioning stone 60 is commonly deemed to have reached the end of its useful life and is replaced with a new conditioning stone before conditioning the planarizing pad 40 again. With appropriate changes in the conditioning process parameters, the same conditioning stone 60 can be used in additional conditioning cycles. Commercial microelectronic component manufacturers, however, do not have at their ready disposal processes for accurately detecting the condition of the conditioning stone 60 and the removal rate of the pad material in situ. The current approach, therefore, is wasteful in that conditioning stones 60 are sometimes discarded before the end of their useful life.
The actuator assembly 74 of the second carrier assembly 70 typically urges the conditioning surface 66 of the stone 60 against the planarizing surface 42 of the planarizing pad 40 with a relatively constant force as the conditioning stone 60 sweeps across the planarizing pad 40. The linear velocity of the conditioning stone 60 with respect to the planarizing pad 40 increases as the conditioning stone 60 moves outwardly from the center of the planarizing pad 40 toward the edge of the planarizing pad 40. This can lead to uneven removal of material from the pad 40, causing the pad 40 to deviate from the ideal planar surface. In many systems, the conditioning stone is moved or “swept” across the surface of the planarizing pad 40 as the planarizing pad 40 and/or the conditioning stone 60 are rotated. To obtain a uniform planarizing pad profile, the rate at which the stone 60 sweeps across the pad 40 may be non-uniform. Establishing a suitable sweep profile for a specific combination of materials in the pad 40, stone 60, and consumables often requires substantial trial and error, which can be unduly expensive and time consuming.